1. Field of the Invention
This invention relates to a device for deflection of a beam, especially a light beam, with an adjustable first beam deflection unit which deflects the beam at a deflection point and with a mirror arrangement which images the deflection point onto a destination point, the device preferably being made for independent deflection of the beam in two spacial directions for use in a two-dimensional scanner. The invention is also directed to a process for deflection of a given beam by turning the mirror surface of a plane mirror from a middle position.
2. Description of Related Art
A device of the above-mentioned type is known, for example, from U.S. Pat. No. 4,997,242 in which an a focal, achromatic mirror arrangement, which consists of two concave mirrors arranged opposite one another, is used to image the first rotary mirror on which the beam is incident onto a second rotary mirror, the axes of rotation of the first and second mirrors being perpendicular to one another in order to be able to independently deflect the beam, which is a light beam, in two spacial directions which are perpendicular to one another. Here, the disadvantage is that the mirror arrangement with the two opposing concave mirrors causes considerable astigmatism depending on the angle of rotation of the first mirror so that, even when using an ideal scanning lens, when the first mirror turns, the foci do not lie on a straight line after the second mirror.
Furthermore, imaging a first rotary mirror onto a rotary second mirror by means of an a focal lens combination is known for purposes of independently deflecting a light beam in two spacial directions perpendicular to one another. The defect here is the possibility of chromatic errors and other imaging errors occurring, especially for large angles.
Furthermore, it is known to arrange two rotary mirrors with axes of rotation that are orthogonal to one another and spatially as close as possible to one another. Here, the disadvantages are that the spatially separate location of the two points of rotation, the limited range of the angles of rotation and the necessity of having to generally use mirrors of different sizes.
The object of this invention is to devise a beam deflection device in which imaging errors are essentially independent of the deflection angle of a first deflection unit, and furthermore, are as small as possible.
Additionally, it is an object of the invention to provide a process for deflection of a given beam by turning the mirror surface of a plane mirror from a middle position, the axis of rotation of the plane mirror lying outside the mirror surface and still an offset of the deflected beam as small as possible occurring depending on the deflection angle.
The first object is achieved in accordance with the invention by a device in which the mirror arrangement is made rotationally-symmetrical at least over a certain range of deflection angles, the deflection point and the destination point being located on or near the axis of rotational symmetry of the mirror arrangement.
In this approach, it is advantageous that, by the rotationally symmetrical arrangement of the mirror arrangement with respect to the deflection point and the destination point, the optical properties of the mirror arrangement, and thus, of the entire device, are essentially independent of the deflection angle of the beam at the deflection point, so that the beam position at the destination point is invariant with respect to the beam deflection angle determined by the first beam deflection unit and only the direction of the beam at the destination point corresponding to the deflection angle at the deflection point changes.
Preferably, this invariance property of the device according to the invention is used to deflect the beam independently in two spacial directions by a second adjustable deflection unit being located at the destination point.
The second object is achieved by a process in which, for deflection of a given beam by means of rotation of the mirror surface of a plane mirror from a middle position, the location of the axis of rotation of the plane mirror being stipulated with respect to the plane mirror and lying outside the mirror surface, the location of the axis of rotation with respect to the given beam is determined by the fact that, for different points of incidence of the beam on the mirror surface for certain angles of rotation of the mirror, the intersection points of the beams which have been reflected at these angles of rotation and which are extended backwards are determined, with the beam which has been reflected in the middle position and which is extended backwards such that, for each point of incidence, a set of intersection points is obtained, from these sets of intersection points, that set of intersection points is determined which shows the minimum variation of intersection points with the angle of rotation with respect to a suitable criterion, and the location of the axis of rotation of the mirror is chosen according to the point of incidence which belongs to this set of intersection points.
It is advantageous that the axis of rotation of the mirror can be placed, for example, through the center of gravity of the mirror, and thus, a location of the mirror can be found in which only a small offset of the reflected beam occurs when the deflection angle changes.
In the following, several embodiments of the invention are detailed by way of example using the attached drawings.